The present invention relates to semiconductor packaging, specifically to heat sinks for packaging electronic components.
As is well known in the art, active semiconductor components generate heat during switching. As integrated circuit devices have grown in size, with higher and higher levels of device density on each chip, the specific power consumption of these devices has increased substantially. The increase in specific power consumption, and concomitant power dissipation, creates the need for packaging arrangements that can conduct away sufficient amounts of the power dissipated by the chip to maintain sufficiently low device junction temperatures to achieve reliable, long-life circuit operation. Although historically heat dissipation through the component leads was adequate to maintain device temperature for low density integrated circuit devices, modern high density integrated circuits require heat sinks or heat spreaders in order to maintain device temperature at an acceptably low level.
Conventionally, heat sinks for semiconductor packages comprise stampings, extrusions, or photochemically machined components that are brazed or bonded to the chip to increase the dissipation of heat by convection directly to the surrounding air. The materials used for semiconductor heat sinks are most commonly stainless steel, copper, and aluminum. Of these, aluminum is most often preferred due to its low thermal inertia and low cost. Aluminum heat sinks, however, in most cases must be protected from corrosion typically by application of an electrolytic treatment such as electroplating or anodizing. Aluminum and magnesium are particularly well suited to anodizing, but other metals such as zinc, beryllium, titanium, zirconium, and thorium also respond to anodic treatment. Anodizing is similar to electroplating in that the part to be treated is immersed in a chemical bath and an electric current passed through the part. Anodizing differs from electroplating, however, in two ways. In electroplating, the work piece is made the cathode and the metallic coatings are deposited on the work piece. In anodizing, the work piece is made the anode, and its surface is converted to form an oxide that is integral with the metal substrate. The metallic portion of the electrolyte container is made the cathode.
Anodizing of small parts with certain shapes can be conducted using a perforated basket. Anodizing of the flat heat sinks most commonly used in integrated circuit packaging, however, currently must be carried out with the heat sinks individually racked, usually by hand. The rack serves to conduct the current from the current source to the heat sink and also to hold the heat sinks firmly under the agitated electrolyte. The process of hand loading the heat sinks onto a rack for anodizing adds substantial cost to the anodizing process. Moreover, the area of the rack that contacts the part must be sufficiently large to carry the unit amperage required for the anodizing process, and the part must be positioned to avoid gas pockets and to ensure positive solution sweep over the part. Accordingly, rack marks are left on the part in varying sizes. These rack marks areas have no anodic film, and therefore, are open to attack by corrosive agents.
Accordingly, what is needed is a semiconductor heat sink that is suitable for anodizing or the application of other coatings that can be processed without racking and which will produce a part that is free from unprotected rack marks.
The present invention comprises a unitary precision stamping consisting of a rail to which are attached a plurality of heat sinks. Each of the heat sinks is attached to the rail by a bridge region of reduced thickness. The heat sinks, supported by the rail member, may be subjected to the anodizing treatment or other surface coating treatment while still attached to the rail. Once the anodizing or other surface treatment is completed, the heat sinks can be separated from the rail member conventional means such as trimming or bending the heat sinks away from the rail to cause the area of reduced thickness to fracture. The area of reduced thickness may run the full length of the heat sink or may comprise a necked region having a lateral dimension that is less than the corresponding lateral dimension of the heat sink. Optimally, the thickness of the area of reduced thickness is chosen to correspond to the design thickness of the anodized coating. Since the anodized coating penetrates the surface of the part being anodized, by selecting the appropriate thickness for the region of reduced thickness, the anodized coating can be made to fully penetrate the bridge region such that when the heat sink is separated from the rail member, the anodic film completely covers the heat sink, without the presence of bare spots.